1. Field of the Invention
This invention relates generally to non-contacting mechanical end face seals and more specifically to seals which are used in applications where shaft rotational speeds are slow, as in mixers, agitators, and reactors and certain specialty slow speed pumps.
2. Background Art
Mechanical end face seals of the non-contacting type have become high technology items utilizable in a variety of industries. These type of seals are designed with a great deal of care and attention to the materials, shapes, dimensions and tolerances of the component parts. Such attention to detail in the design is necessary in order to accommodate a great number of parameters and characteristics, any of which parameters and characteristics will affect the efficiency or operation of these seals.
Minor changes or alterations to any one of about 100 physical features of a seal, its components or its sealing environment may, and in most cases will, result in a change in the characteristics of a seal, such as sealing capability, wear, endurance and/or seal integrity. In most cases, it is desirable for seals of this type to operate without maintenance and trouble for extensive periods of time, preferably on the order of years. Failure of such seals often leads to plant upset and even shutdown. The seals are used in machinery which cannot be shut down for long without severely impairing the operation and efficiency of, for example, a large mixer used in a processing plant.
Seals of this type may be utilized in single, dual or double seal arrangements. The dual or double arrangement of the two seals are normally spaced apart axially along a shaft passing through an aperture in a housing. Dual seals include a barrier fluid in the intermediate chamber defined by the housing and by the two seals, the buffer fluid being neutral relative to the environment, to the product being sealed and to the materials comprising the seal elements.
An arrangement of double seals is described in commonly owned U.S. Pat. No. 5,375,853, which is hereby incorporated by reference. U.S. Pat. No. 5,375,853 illustrates and describes a dual non-contacting type seal arrangement having a relatively inert gas, such as nitrogen, as the buffer fluid. The buffer gas is at a high pressure in excess of the pressure of the sealed liquid within the housing.
For particular applications, such as in mixers, severe or extreme conditions arise, for example, the sealed fluid may have a high viscosity. In a mixer having a very long shaft, the shaft runout is usually much greater than in an application in which the shaft is bearing contained and is of shorter length. For seals in use with mixers of this type, extreme shaft runout must be accommodated or the seal will lose its effectiveness and possibly its integrity after a short period of use.
It has been found that seals of the type described and illustrated in U.S. Pat. No. 5,375,853 operate well in most applications. However, because the seal arrangements made in accordance with that patent were developed for predetermined rotational speed and pressure conditions, limitations arise in the event that the shaft rotational speed is decreased below a presently predetermined limit of about 400 feet/minute.
Another requirement is that any seal must provide liftoff capability, which may become problematic when the shaft rotates at low rotational speeds. In normal applications, a gas lubricated seal is mounted on a shaft that rotates well above 1,200 rpm. At such speeds, pumping action from conventional spiral grooves operates adequately to provide liftoff between the seal faces during normal operation, and the seal operates to effectively limit leakage.
Accommodations must be provided in the seal face geometry and in the spiral groove geometry for applications in which the shaft operates at low rotational speeds. Liftoff between the seal faces is preferable at an early period in the commencement of shaft rotation. A seal having decreased liftoff capability, or liftoff which ensues well after the shaft has begun rotating at a minimum speed, sometimes results in seal face contact during operational conditions. The period during shut down or start up of the device utilizing the seal is especially liable to present conditions in which seal faces contact. Such face contact is undesirable because it leads to excessive wear and to ineffectiveness of the seal soon after a short period of normal operation.
What is thus necessary is a seal which can provide seal liftoff capability at low rotational speeds and can accommodate greater than normal shaft runouts so as to enable use of the seal in a great many heretofore difficult applications, such as in mixers, agitators and reactors.